Certain types of cathode ray tube (CRT) displays consist of distinct separated lines of information. Two modalities presently using this method for displaying continuous tone or grey scale images are raster-scan systems and real time ultrasound displays. In raster-scan systems, the electron beam in the CRT traces a sequence of horizontal or vertical lines which cover the tube face. This pattern is continually repeated while the beam intensity is modulated to produce an image. If viewed from a sufficient distance or if the electron beam is sufficiently out of focus, the lines disappear and the appearance of a continuous image is produced. This is generally known as a television display. The number of scan lines has been standardized at 525 or 650, although greater or fewer lines may be used so that more or less information is displayed during a single illumination of the screen. The entire screen usually is rescanned from 30 to 40 times per second so that "flicker" is not apparent to an observer.
In real time ultrasound displays, the pattern is again made up of a modulated beam repeating a sequence of lines. However, in real time ultrasound displays these lines may be parallel, or can extend radially from a point producing a sector-shaped or circular shaped pattern.
When resolution of fine detail is important, as it is in medical applications, the spaces between the raster or scan lines break up the image, interfering with the observation of fine detail. It is highly desirable that a true continuous tone be presented to the observer so that the image has no observable lines. This can be accomplished by greatly increasing the number of horizontal lines. However, such is extremely expensive as it requires a much wider frequency spectrum in the electronics producing the image. In applications where it is important for an observer to obtain detailed information from these displays, a photograph of the CRT image normally is taken so that a number of images can be observed at different locations and times. These photographs are also retained as permanent records of the displayed information. As with direct visual viewing, it is desirable that the recorded image appear to be a solid continuous tone rather than made up portions of parallel lines of different brightness. With photographs the aforementioned distinct lines making up the image will appear as a continuous tone if the image can be blended as the photographic film is being exposed. This increases the effective resolution and therefore analytical value of the displayed information on the photographs.